One runner, a Rhode Island state trooper, said he saw at least two dozen people with very serious injuries, including missing limbs.

About two hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.

The Boston Marathon said that bombs caused the two explosions and that organizers were working with authorities to determine what happened. The Boston Police Department said two people were killed and 23 others injured.

Competitors and race volunteers were crying as they fled the chaos. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners. Authorities went onto the course to carry away the injured while stragglers in the 26.2-mile race were rerouted away from the smoking site.

Roupen Bastajian, a 35-year-old state trooper from Greenville, R.I., had just finished the race when they put the heat blanket wrap on him and he heard the first blast.

“I started running toward the blast. And there were people all over the floor,” he said. “We started grabbing tourniquets and started tying legs. A lot of people amputated. … At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing.”

A Boston police officer was wheeled from the course with a leg injury that was bleeding.

Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.

— Associated Press

12:30 p.m. BOSTON — Two explosions shattered the euphoria of the Boston Marathon finish line today, sending authorities out on the course to carry off the injured while the stragglers in the 26.2-mile trek from Hopkinton were rerouted away from the smoking site of the blasts.

Competitors and race organizers were crying as they fled the chaos. Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners.

“There are a lot of people down,” said one man, whose bib No. 17528 identified him as Frank Deruyter of North Carolina. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg. A Boston police officer was wheeled from the course with a leg injury that was bleeding.

About three hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another thunderous explosion could be heard a few seconds later.

Runner Laura McLean of Toronto said she heard two explosions outside the medical tent.

“There are people who are really, really bloody,” McLean said. “They were pulling them into the medical tent.”

Cherie Falgoust was waiting for her husband, who was running the race. “I was expecting my husband any minute,” she said. “I don’t know what this building is … it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don’t know what it was. I just ducked.”